Personal watercraft have become very popular in recent years. This type of watercraft is quite sporting in nature and carries a rider and possibly one or two passengers. A relatively small hull of the personal watercraft commonly defines a riders' area above an engine compartment. A two-cycle internal combustion engine frequently powers a jet propulsion unit which propels the watercraft. The engine lies within the engine compartment in front of a tunnel formed on the underside of the watercraft hull. The jet propulsion unit is located within the tunnel and is driven by a drive shaft. The drive shaft usually extends between the engine and the jet propulsion device, through a wall of the hull that forms the front gullet portion of the tunnel.
Personal watercrafts often employ an in-line, multi-cylinder, crankcase compression, two-cycle engine. The engine conventionally lies within the engine compartment with the in-line cylinders aligned along the a longitudinal axis of the watercraft hull. An exhaust system communicates with the cylinders of the engine and extends to a discharge located near the stern of the watercraft. In this manner, exhaust gases are expelled from the watercraft hull and engine compartment.
Exhaust systems within personal watercraft typically employ some form of manifold exhaust system to account for weight and cost concerns. Prior exhaust manifold systems use one runner for each cylinder which communicates with the exhaust port of the respective cylinder. At a point downstream of the ports, the runners merge together. Because each cylinder discharging exhaust into the exhaust manifold does so at different times, it is important to be able to control the respective pressure pulses associated with the discharges by tuning the length of the exhaust runners. The desired tuning is difficult to achieve, however, due to the cramped space provided within the engine compartment of a personal watercraft.
Additionally, due to the cramped space available within the personal watercraft engine hull, manufacturers have begun to utilize an inclined cylinder engine arrangement. The weight of the exhaust system and the corresponding cylinders, which are then placed off-center, can create balancing problems. In the past, the engine weight distribution was sought to be centered through a centrally located engine. The inclined cylinders, however, do not readily allow a personal watercraft employing an inclined engine to be balanced side-to-side.
Further, water constantly enters the engine compartment of a small watercraft through the air vents due to the sporting nature of the watercraft. Although bilge pumps operate to evacuate any water that accumulates within the compartment, such water may tend to splash from side to side before evacuated when the personal watercraft is being maneuvered. The splashing water can compromise engine performance when sensitive engine components become wet. For instance, exhaust control valves and ignition systems performance typically are hindered when operating in a wet environment.